1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
2. Description of the Related Art
At present, there has been developed a charge-trap-type nonvolatile semiconductor memory device which uses, as a charge-storage layer, a charge-storage insulating film for charge trap (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2004-158810). In this charge-trap-type nonvolatile semiconductor memory device, the charge, which has been injected in the charge-storage layer via a tunnel insulating film, is trapped at a trap level in the charge-storage insulating film, and thereby the charge is accumulated in the charge-storage insulating film. As a typical charge-trap-type nonvolatile semiconductor memory device, there is known a MONOS type or SONOS type nonvolatile semiconductor memory device, wherein a silicon nitride film, for instance, is used as the material of the charge-storage insulating film.
In the above-described charge-trap-type nonvolatile semiconductor memory device, there is proposed a tunnel insulating film having a multilayer structure (ONO structure) comprising a silicon oxide film, a silicon nitride film and a silicon oxide film, in order to increase the charge erase speed (see, e.g. Jpn. Pat. Appln. KOKAI Publication No. 2006-216215).
However, in the nonvolatile semiconductor memory device having the above structure, a defect occurs at the interface between the silicon nitride film and silicon oxide film due to a stress occurring from the differences in inter-lattice distance and film expansion coefficient between the silicon nitride film and silicon oxide film. This defect becomes a trap site of electrons and holes. The trapped electron or hole leaks into the semiconductor substrate, causing deterioration in charge retention characteristics of the charge-storage insulating film. Thus, it cannot necessarily be said that there has been proposed a nonvolatile semiconductor memory device having both excellent charge erase characteristics and excellent charge retention characteristics.